The present invention relates to the field of display signs which employ large numbers of individually energized light emitting diodes lights that form the pixels in an image, and more particularly, power supplies for such signs which limit the junction temperature of the light emitting diode.
A display sign which employs a matrix of light emitting diode (LED) lights requires a relatively large power supply to provide the current necessary to energize them. For example, a 7 by 80 matrix of such lights may require 10 amperes at over 5 volts to energize 50% of the lights at any one time at the optimal brightness. This load on the power supply changes dramatically if the image requires the energization of more or fewer lights at any one time, and these load variations will cause swings in the power supply output voltage and current. While the output can be regulated to reduce such voltage and current swings, the additional regulator circuit components increase the power supply cost from 30% to 50%, and this is commercially unacceptable.
Two problems are presented when an unregulated power supply is used to energize an LED display. First, if too many lights are energized at one time, the current limit of the power supply is exceeded and a protective fuse is blown. This can be avoided by increasing the capacity of the power supply, but such a solution is also too expensive, particularly in view of the fact that this extra capacity is rarely needed in typical applications.
At the other extreme, if very few lights are energized simultaneously, the power supply voltage rises and increased current flows through the energized LEDs. This causes a rise in the LED's junction temperature which has been shown to increase their degradation rate, and thus, shorten their useful life. This is a situation that occurs often in images and must be taken into consideration by the power supply designer. One solution is to reduce the maximum current at peak voltages by using a larger series resistor, but this resistor also reduces the brightness of the display to unacceptable levels when large numbers of lights are energized and the power supply voltage drops.